mimxrt: Add some documentation for the MSC mode.

robert-hh · robert-hh · commit f24a814e8eac · 2022-06-26T22:13:13.000+02:00
diff --git a/docs/mimxrt/quickref.rst b/docs/mimxrt/quickref.rst
@@ -537,6 +537,46 @@ port and LAN(1) for the 1G port.
 
 For details of the network interface refer to the class :ref:`network.LAN <network.LAN>`.
 
+
+External drive mode
+-------------------
+
+On some boards a mode is enabled, that mounts the board's internal file system as
+external USB drive, called MSC support. Data of that drive can be accessed and changed by the PC.
+In that state, access to the internal drive by the MicroPython is limited to
+readonly mode, avoiding file corruption. Changes made by the PC to the file system may not be visible
+at the board until the drive is ejected by the PC or a soft reset of the board
+is made.
+
+To enable write access to by the board, eject the drive at the PC **and** perform
+a soft-reset on the board, either by pushing Ctrl-D at REPL or calling machine.soft_reset().
+
+The external drive mode can be enabled of disabled by calling mimxrt.usb_mode(). Examples::
+
+    # disable MSC support
+    import mimxrt
+    mimxrt.usb_mode("vcp")
+
+::
+
+    # enable MSC support
+    import mimxrt
+    mimxrt.usb_mode("vcp+msc") # enable VCP and MSC support
+
+These command must be placed into boot.py. They get effective after the next hard reset or
+power cycle.
+
+The readonly state of the local file system access can be told by an IOCTL call of the file
+system's block device. ::
+
+    import mimxrt
+
+    bdev = mimxrt.Flash()
+    readonly = bdev.ioctl(7, 0)
+
+If the drive is in readonly state, bdev.ioctl(7, 0) returns `True`.
+
+
 Transferring files
 ------------------
 
